Hi there! I'm trying to render some High quality character with HDRP in unity. 
But when i import a character with a clothing on another piece of clothing (bra and skirt) (t-shirt and jacket) i have this issue.

Any suggestion?

Hi Gamit,

You can manually hide the unwanted surface of clothes in CC, and then check "Delete Hidden Mesh" when exporting FBX file.

Reference:
http://manual.reallusion.com/Character_Creator_3/ENU/3/Content/Character_Creator_3/3/08_Cloth/Hiding_Specific_Inner_Meshes.htm
http://manual.reallusion.com/Character_Creator_3/ENU/3/Content/Character_Creator_3/3/08_Creating_Custom_Assets/Showing_Hiding_Meshes_with_Brush.htm
http://manual.reallusion.com/Character_Creator_3/ENU/3/Content/Character_Creator_3/3/17_Export/Exporting_FBX_Avatars.htm